1. Field of the Invention
The present invention relates generally to the vaporization of solids. More particularly, the present invention relates to an apparatus and method for vaporizing solid precursor material and injecting the vaporized material into a chemical vapor deposition reactor.
2. Description of the Invention Background
Chemical vapor deposition (CVD) is known as a technique for forming solid films on a substrate by the reaction of vapor phase chemicals near and preferably on the surface of the substrate to produce a solid film. CVD techniques have been particularly useful in the microelectronics industry because CVD techniques can be used to reliably produce extremely thin films, or layers, having good surface coverage characteristics and structural uniformity. In the production of electronic devices, CVD techniques are used to selectively deposit layers on a large silicon wafer-shaped substrate, or wafer, to form a plurality of complex electronic circuit elements separated by narrow streets. The wafers are then cut along the streets to separate the individual elements into chips, or dies, and leads are attached to form the electronic devices.
In general, CVD techniques involve the delivery of gaseous reactants to the surface of a substrate where chemical reactions takes place under temperature and pressure conditions that are favorable to the thermodynamics of the desired reaction. The type and composition of the layers that can be formed using CVD is limited by the ability to deliver the reactants or reactant, precursors to the surface of the substrate. Various liquid reactants and precursors, are successfully used in CVD applications by delivering the liquid reactants in a carrier gas. In liquid reactant CVD systems, the carrier gas is typically bubbled at a controlled rate through a container of the liquid reactant so as to saturate the carrier gas with liquid reactant and the saturated carrier is then transported to the reaction chamber.
Analogous attempts have been made to deliver solid reactants to a CVD reaction chamber, but with much less success. In the solid delivery devices, the carrier gas is passed through a chamber containing volatile solid reactants or precursors at conditions conducive to the volatilization, or vaporization, of the solid. The carrier gas mixes with the vaporized solid and the vaporized solid is transported with the carrier gas to the reaction chamber. However, this procedure has been unsuccessful in reliably and reproducibly delivering solid precursor to the reaction for a number of reasons. The major problems with the technique are centered on the inability to consistently contact the carrier gas with the solid material due to the fact that, as a solid is vaporized, the amount of solid changes with time as well as the shape and morphology of the remaining solid. Because of the difficulty in delivering a predictable amount of solid precursor to a CVD reaction chamber, the use of solid reactants is currently very limited and the reactions that would normally be performed using solid precursors are currently performed using alternative liquid and gas precursors or reactants.
The relative inability to deliver solid precursor has limited the types of the material that can be deposited using CVD techniques. One apparatus that has been developed to more consistently deliver solid precursor to a metal oxide CVD (MOCVD) reaction chamber for producing superconducting thin film is disclosed in U.S. Pat. No. 5,447,569 issued to Hiskes. In the Hiskes apparatus, the solid material is packed into a tube, which is vertically positioned and movable by means of a magnetic support structure within one leg of a quartz U-shaped conduit. The other leg of the conduit is attached to the MOCVD reaction chamber. The tube is plugged at the bottom end (nearest to the reaction chamber) and is provided with a plurality of longitudinal slits extending over the length of the tube. The vaporization of the solid material within the tube is controlled by moving the tube through a band of heaters operated so as to only locally heat and vaporize the material in the immediate vicinity of the heaters. The vaporized material must exit the tube perpendicularly to the longitudinal axis of the tube and via the slits because the end of the tube is plugged. As stated in the Hiskes patent at column 6, lines 63-64, the key feature is that the feed tube moves vertically downward into the heating zone. This feature is required so that nonvolatile constituents in the solid material move downwardly away from the unvaporized material and are retained in the plugged end, thereby requiring that the vaporized material exit perpendicularly to the longitudinal axis of the tube.
The technique of the Hiskes patent provides certain improvements over the prior art methods of delivering solid precursors to a CVD reaction chamber. However, the apparatus and technique of the Hiskes patent are somewhat limited in application resulting from the required orientation of the tube and the necessity of having slits in the tube to allow the vaporized material to exit the tube. For instance, solid precursors that are unstable in oxygenated environments can not be easily provided for in the slit tube design. Also, the vertical orientation of the tube combined with the vaporization of the material from the bottom of the tube requires that the material be fairly tightly packed into the tube to prevent unvaporized material from falling within the tube as the material located below it in the tube is vaporized, thereby removing the support for the above material. This effect will be accentuated by the heating of the material from the outermost radius of the tube toward the central axis of the tube that will tend to initially vaporize the material nearest to the tube wall that provides support for the cross section of the tube, when the powdered solid is packed in the tube. The potential instability in the tube material can be diminished through the use of nonvolatile solid dispersants. The dispersants act as a support structure for the volatile solid precursors in the tube, so that when the volatile solids are vaporized, the aforementioned structural instabilities in the powder do not result. While the dispersants are beneficial in providing structural support, the benefit is not derived without a substantial penalty. Because the dispersant must occupy a sufficient volume of the tube to act as a support, the amount of volatile solid that can be loaded into the tube is greatly diminished requiring substantially longer tube spans to provide a given mass of volatile material. For example, the Hiskes patent at column 6, lines 30-32 provides for typical volatile solid percentages ranging from 1-10%. In addition, the heat input necessary to vaporize the volatile material will be significantly more than the heat of vaporization due to the need to heat the nonvolatile dispersant to the vaporization temperature along with the volatile material. Also, the solid that has been vaporized within the nonvolatile dispersant must diffuse through the nonvolatile material to exit the tube via the longitudinal slits in a direction perpendicular to gravity and therefore unaided by the buoyancy force, although the application of a vacuum downstream of the tube should be a compensating factor. An additional complicating factor is potential for condensation and solidification of the volatile solid near the entrance to the heating section, where vaporized solid is contacted by cold carrier gas. The condensed solid may not necessarily be vaporized lower in the tube, unless the solidification takes place on a wall, but may accumulate in the conduit.
The present invention is directed to an apparatus and method for vaporizing solid precursor and reactant material and delivering the vaporized material to a CVD reactor which overcomes, among others, the above-discussed problems so as to provide a more efficient method for using solid precursors and reactants in the formation of thin layers on substrate using CVD techniques.